High peripheral temperatures in king penguins while resting at sea: thermoregulation versus fat deposition

Abstract

Marine endotherms living in cold water face an energetically challenging situation. Unless properly insulated these animals will lose heat rapidly. The field metabolic rate of king penguins at sea is about twice that on land. However, when at sea, their metabolic rate is higher during extended resting periods at the surface than during foraging, when birds descend to great depth in pursuit of their prey. This is most likely explained by differences in thermal status. During foraging peripheral vasoconstriction leads to a hypothermic shell, which is rewarmed during extended resting bouts at the surface. Maintained peripheral perfusion during rest in cold water, however, will greatly increase heat loss and, therefore, thermoregulatory costs. Two hypotheses have been proposed to explain the maintenance of a normothermic shell during surface rest: (1) to help the unloading of N2 accumulated during diving; and (2) to allow the storage of fat in subcutaneous tissue, following the digestion of food. We tested the latter hypothesis by maintaining king penguins within a shallow sea water tank, while we recorded tissue temperatures at four distinct sites. When released into the tank during the day, body temperature of king penguins immediately declined. However, during the night periodic rewarming of abdominal and peripheral tissues occurred, mimicking temperature patterns observed in the wild. Temperatures, particularly in the flank, also depended on body condition and were higher in ‘lean’ birds (after 10 days of fasting) than in ‘fat’ birds. While not explicitly tested, our observation that nocturnal rewarming persists in the absence of diving activity during the day, does not support the N2 unloading hypothesis. Rather, differences in temperature changes throughout the day and night, and the effect of body condition/mass supports the hypothesis that tissue perfusion during rest is required for nutritional needs.